The quasi-one-dimensional nanostructures, such as carbon nanotubes or semiconductor nanowires, are used in particular in the field of electronics (for example, thin-film transistors), in the field of optoelectronics (for example, solar cells, electroluminescent diodes), and as sensors because of their original properties (optical, electronic, thermal, mechanical) and their large specific surface area.
These nanostructures are obtained by being grown or deposited on a substrate that, in the field of optoelectronics, is preferentially both transparent and conductor of electricity. Such a substrate may be obtained by applying a thin-film layer of metal oxide as, for example, tin oxide (SnO2) or tin-doped indium oxide (ITO, “indium tin oxide”), on a substrate of glass, polymer material, or other material. Hereinafter, such a substrate will be referred to as “metal oxide substrate”.
Most of the nanostructures for which the direct growth on such metal oxide substrates is mentioned in the prior art are formed of metal oxides.
As for the nanostructures made from other semiconductors (silicon, germanium, gallium arsenide) or carbon, in most cases, they grow on single-crystalline silicon substrates, and are optionally transferred afterwards to another substrate.
The prior art processes of producing nanostructures conventionally comprise a first step of forming metal aggregates on the substrate, which serve to catalyze the nanostructures growth.
Several methods are known for positioning catalyst particles (or metal aggregates), such as the lithography method, the use of porous membranes, the deposition of metal colloids, as well as the evaporation and annealing of a thin-film metal layer.
Most of such methods for producing nanostructures on a substrate require heavy equipments, and can not be applied to large surfaces. They can neither be carried out in situ, except for the method of evaporating the metal layer, which, on the other hand, requires a complex installation.
Once the metal aggregates or catalysts are formed on the substrate, the later is transferred to a reactor in order to carry out a second step, which is the step of growing the nanostructures. Such transfer causes pollution or oxidation of the catalysts in contact with air, as well as loss of time (operations of loading/unloading, pumping, etc. . . . ).
During this second step, the nanostructure growth is carried out in vapour phase according to the vapour-liquid-solid (VLS) mechanism, known from: Wagner, R. S. & Ellis, W. C. “Vapor-liquid-solid mechanism of single crystal growth”, Applied Physics Letters, 1964, 4, 89-90, or the vapour-solid-solid (VSS) mechanism, known from: Arbiol, J.; Kalache, B.; Roca i Cabarrocas, P.; Ramon Morante, J. & Fontcuberta i Morral, A., “Influence of Cu as a catalyst on the properties of silicon nanowires synthesized by the vapour-solid-solid mechanism”, Nanotechnology, 2007, 18, 305606. The nanostructure growth is generally carried out from gaseous precursors by the method of chemical vapour deposition (CVD). The nanostructure growth is catalyzed by the metal aggregates.
Most of the known processes use single-crystalline substrates (generally silicon), so as to obtain an epitaxial growth.
A growth process by the method of CVD deposition on metal oxides for carbon nanotubes is also known from the prior art (Miller, A. J.; Hatton, R. A.; Chen, G. Y. & Silva, S. R. P., “Carbon nanotubes grown on In2O3: Sn glass as large area electrodes for organic photovoltaics”, Applied Physics Letters, AIP, 2007, 90, 023105).
As for the semiconductor nanowires, the most known catalyst for the growth of such nanowires is gold, as disclosed in the document FR 2,873,492, with which the temperature is generally higher than 500° C. The use of gold as a catalyst causes electronic defaults in silicon.
Other metals, each having drawbacks, have also been tested, for example copper, with which the temperature window for the growth of nanowires is comprised between 600 and 650° C., aluminium, which oxides very rapidly and which thus requires an ultra-vacuum transfer of the samples, and nickel, with which the growth of carbon nanotubes is possible from 600° C., but is very slow under 700° C.
There is no known method that permits to grow nanostructures of a chemical element such as silicon directly on a metal oxide substrate.